A differential gear device including differential gears is one example of a power transmission device for a vehicle to transmit drive power or driving force from a power source such as an engine to drive wheels. As shown in FIG. 10, a differential gear device 300 includes a differential case 301 (hereinafter, also referred to simply as a “case”), side gears 314 and 315 and pinion gears 312 and 313 mounted inside the differential case 301, and a ring gear 302 fixedly attached on the outer peripheral surface of the differential case 301. An annular flange 310 is formed on the outer peripheral surface of the conventional differential case 301, and the ring gear 302 is fastened to this flange 310 with bolts.
However, to fasten the ring gear 302 to the flange 310 of the differential case 301 with bolts, it is necessary to form screw holes in the ring gear body, and to form bolt through holes in the flange 310 of the differential case 301. The bolts need to be fastened after the ring gear 302 is fitted on the differential case 301, and the fastening torque of each bolt 311 need to be inspected. The large number of steps in these machining, assembling, and inspection processes was the cause of increase in cost of the vehicle power transmission devices.
The differential case 301 is conventionally made of spherical graphite cast iron, as casting allows formation of complex shapes, while the ring gear 302 is made by machining or forging from chromium molybdenum steel or the like as it is a component that requires high strength. Therefore, in welding the differential case 301 and the ring gear 302 to securely join them together, there was a problem of high cracking tendency in weld portions as these two parts are made of different materials.
The ring gear 302, which is an annular component, needs to be welded all around, with the start and finish ends of the weld bead being lapped, and this lapping portion of the weld bead is particularly prone to cracking as it is subjected to heat twice and suffers larger thermal contraction than other parts.
Patent Document 1 discloses a method of inspecting weld portions for cracks, wherein a portion of the weld portion that undergoes melting and solidifying processes or a portion near a heat-affected zone is irradiated with a laser beam, and the strain amount is dynamically measured based on changes in a speckle pattern, to detect cracks in the weld portion from changes with time in the dynamic strain amount. With this method, the strain on the surface of the weld portion is measured in a non-contact manner to detect high-temperature cracking in the weld portion, which is indicated by an interrupted portion of a strain curve that represents changes with time in the strain amount.
Patent Document 2 discloses a deformation monitoring device for welded structures for monitoring welding deformation of welded structures, characterized by having a displacement sensor that measures a displacement on the surface of a welded structure; a deformation amount calculation device that calculates an amount of deformation of the welded structure based on data from the displacement sensor and positional information of displacement measurement points; a thermometer that measures a surface temperature of the welded structure; a temperature distribution calculation device that estimates a temperature distribution of the welded structure based on data from the thermometer and positional information of temperature measurement points; and an evaluation device that evaluates welding deformation of the welded structure during the welding. The evaluation device includes a first computing unit that estimates thermal deformation amount caused by linear expansion of the welded structure from the temperature distribution of the welded structure obtained by the temperature distribution calculation device; a second computing unit that computes a true amount of welding deformation by subtracting the estimated thermal deformation amount obtained by the first computing unit from the amount of deformation of the welded structure obtained by the deformation amount calculation device; a database that has tolerance values of welding deformation amount stored therein in advance for each welding process; and a determination unit that determines whether or not the welding deformation is permissible by comparing the true amount of welding deformation determined by the second computing unit and the tolerance value of the welding deformation amount. This method can determine a true amount of welding deformation in the weld portion caused by elastic and plastic deformation by subtracting thermal deformation amount caused by linear expansion resulting from a temperature distribution from the deformation amount that can be directly observed by measurement, and thus allows monitoring of deformation during a welding process.